The string type is an alias for the System.String class. It is derived from object type. String is just a class name. String is a reference type String is not a keyword and it can be used as Identifier. string is a keyword and cannot be used as Identifier.
It is a single deploy able unit that contains all the information about the implementation of classes , structures and interfaces
It is a logical group of related classes and interfaces and that can be used by any language targeting the .net framework.
A class is the generic definition of what an object is. A Class describes all the attributes of the object, as well as the methods that implement the behavior of the member object. In other words, class is a template of an object.